U.S. patent application number 11/915359 was filed with the patent office on 2008-08-14 for optical integrated circuit comprising a light guide forming at least one optical separation.
This patent application is currently assigned to PHOTLINE TECHNOLOGIES. Invention is credited to Nicolas Grossard, Jerome Hauden, Henri Porte.
Application Number | 20080193077 11/915359 |
Document ID | / |
Family ID | 34955279 |
Filed Date | 2008-08-14 |
United States Patent
Application |
20080193077 |
Kind Code |
A1 |
Grossard; Nicolas ; et
al. |
August 14, 2008 |
Optical Integrated Circuit Comprising a Light Guide Forming at
Least One Optical Separation
Abstract
An optical integrated circuit with waveguide separation on a
substrate includes at least one separating unit, including an
optical input/output interface in relation with an external light
wave guide, the interface extending in the circuit through an
optical guiding input section extended by at least two optical
guiding branches mutually spaced apart substantially symmetrically
relative to the general direction of the input section. The input
section includes as many optical guides as branches, adjacent input
section optical guides being substantially rectilinear and mutually
parallel, two adjacent optical guides of the input section being
separated by an aperture of width D, the refractive index of the
opening being lower than that of the optical guides, each input
section optical guide having a determined width We1, each branch
optical guide exhibiting a width increasing in the direction away
from the input section from the width We1 up to a determined width
Ws.
Inventors: |
Grossard; Nicolas;
(Besancon, FR) ; Hauden; Jerome; (Besancon,
FR) ; Porte; Henri; (Serre Les Sapins, FR) |
Correspondence
Address: |
Young & Thompson
745 S. 23rd Street., Second Floor
Arlington
VA
22202
US
|
Assignee: |
PHOTLINE TECHNOLOGIES
Besancon
FR
|
Family ID: |
34955279 |
Appl. No.: |
11/915359 |
Filed: |
May 24, 2006 |
PCT Filed: |
May 24, 2006 |
PCT NO: |
PCT/FR2006/050480 |
371 Date: |
November 23, 2007 |
Current U.S.
Class: |
385/14 |
Current CPC
Class: |
G02B 2006/1215 20130101;
G02B 6/125 20130101 |
Class at
Publication: |
385/14 |
International
Class: |
G02B 6/12 20060101
G02B006/12 |
Foreign Application Data
Date |
Code |
Application Number |
May 25, 2005 |
FR |
0551376 |
Claims
1-9. (canceled)
10. An optical integrated circuit with waveguide separation on a
substrate (3), the circuit comprising at least one optical
separating unit, the unit comprising an optical input/output
interface (2) at the edge of the substrate intended for being in
relation with an external means for guiding a light wave, the
interface extending in the circuit through an optical guiding input
section of determined length L1 extended by at least two optical
guiding branches (7, 8) mutually spaced apart substantially
symmetrically relative to the general direction of the input
section, characterized in that the input section from the interface
at the edge of the substrate includes as many optical guides
(4a,4b) as there are branches, each of the optical guides being
continuous from the interface at the edge of the substrate up to
its corresponding branch inclusive, the optical guides of the input
section being substantially rectilinear and mutually parallel, two
adjacent optical guides of the input section being separated by an
aperture (9) of determined width D, the refractive index of the
opening being lower than that of the optical guides, each optical
guide of the input section having a determined width We1, and in
that each branch optical guide exhibits a width increasing in the
direction away from the input section from the width We1 up to a
determined width Ws, the widths We1 and D being such that the
guides are monomode at the working wavelength.
11. An optical integrated circuit according to claim 10,
characterized in that the unit includes moreover a transition zone
of length L0 between the interface and the input section, wherein
the transition zone includes optical guides (10a,10b) continuous
with those of the input section (4a,4b), each of the optical guides
of the transition zone having a width increasing in the direction
away from the interface from a determined width We0 up to the width
We1, and in that the opening (9) between two adjacent optical
guides of the transition zone has a width increasing in the
direction away from the interface from a determined width D' up to
the width D.
12. An optical integrated circuit according to claim 10,
characterized in that the variation in width of the optical guides
of the branches is linear in relation to the distance of
propagation.
13. An optical integrated circuit according to claim 10,
characterized in that the length L1 of the input section ranges
between 0 excluded and 10 mm.
14. An optical integrated circuit according to claim 10,
characterized in that in the case of two branches the semi-angle
.alpha. for separating the branches ranges between 0.1.degree. and
0.50.degree. and is preferably about 0.175.degree..
15. An optical integrated circuit according to claim 10,
characterized in that the external guiding means is an optical
fiber (1) bonded to the interface of the optical integrated
circuit.
16. An optical integrated circuit according to claim 10,
characterized in that the substrate of the optical integrated
circuit is made of lithium niobate (LiNbO3).
17. An optical integrated circuit according to claim 10,
characterized in that it comprises a separating unit and that it is
a Y-shaped optical separator/recombinator with at least two
branches.
18. An optical integrated circuit according to claim 10,
characterized in that it comprises two cascaded, head to tail
mounted separating units and that it is an integrated Mach-Zehnder
interferometer with at least two branches.
19. An optical integrated circuit according to claim 11,
characterized in that the variation in width of the optical guides
of the branches is linear in relation to the distance of
propagation.
20. An optical integrated circuit according to claim 11,
characterized in that the length L1 of the input section ranges
between 0 excluded and 10 mm.
21. An optical integrated circuit according to claim 12,
characterized in that the length L1 of the input section ranges
between 0 excluded and 10 mm.
22. An optical integrated circuit according to claim 11,
characterized in that in the case of two branches the semi-angle
.alpha. for separating the branches ranges between 0.1.degree. and
0.50.degree. and is preferably about 0.175.degree..
23. An optical integrated circuit according to claim 12,
characterized in that in the case of two branches the semi-angle
.alpha. for separating the branches ranges between 0.1.degree. and
0.50.degree. and is preferably about 0.175.degree..
24. An optical integrated circuit according to claim 13,
characterized in that in the case of two branches the semi-angle
.alpha. for separating the branches ranges between 0.1.degree. and
0.50.degree. and is preferably about 0.175.degree..
25. An optical integrated circuit according to claim 11,
characterized in that the external guiding means is an optical
fiber (1) bonded to the interface of the optical integrated
circuit.
26. An optical integrated circuit according to claim 12,
characterized in that the external guiding means is an optical
fiber (1) bonded to the interface of the optical integrated
circuit.
27. An optical integrated circuit according to claim 13,
characterized in that the external guiding means is an optical
fiber (1) bonded to the interface of the optical integrated
circuit.
28. An optical integrated circuit according to claim 14,
characterized in that the external guiding means is an optical
fiber (1) bonded to the interface of the optical integrated
circuit.
29. An optical integrated circuit according to claim 11,
characterized in that the substrate of the optical integrated
circuit is made of lithium niobate (LiNbO3).
Description
[0001] The present invention relates to an optical integrated
circuit comprising a light guide forming at least one optical
separation, said circuit being commonly implemented in devices
called separator or recombinator according to the modalities of use
but which may also be implemented in interferometrical devices. It
finds application in optics, in particular for routing light
beams.
[0002] The optical separators/recombinators, in particular of the
type commonly called Y-junction, are essential elements in
integrated optical circuits. These separators are in particular
used as optical power divider in active (Mach-Zehnder type
intensity modulators) and passive integrated components (separators
1 toward N).
[0003] An optical guide is a structure formed at least of a central
portion, called core, with higher refractive index as its
neighbouring sections, called sheathes. The particularity of an
optical guide is to be able to channel light on a determined path.
The parameters of the optical guide (core width, difference in the
refractive index between the core and the sheathes) define the
number of solutions enabling to guide the light. By monomode
optical guiding is meant a single propagation solution and
consequently a single possible transversal distribution of the
guided optical field. In such a case, the guided optical wave is
called fundamental optical mode. When there are several solutions
for the guiding equation, there are then higher order optical modes
whereof the transversal distributions of the optical field are
alternately symmetrical and anti-symmetrical. These higher order
modes propagate at different speeds in the optical guide which may
generate spurious interference phenomena, sources of optical
instabilities. Generally speaking, the operation of the Y-junction
may be interpreted from the evolution of the guided and radiating
optical modes during the propagation of light. It may be referred
to article of H. Yajima, <<Coupled mode analysis of
dielectric planar branching waveguide", IEEE Journal of Quantum
Electronics, vol. 14, n.degree.10, October 1978, for more details
pertaining to this matter.
[0004] A Y-junction may be characterized by several quality
criteria, in particular optical losses, optical balance between the
output branches, space requirements and wavelength stability,
preferably the best possible. Still, for the reasons stated below,
it is difficult to obtain good features for all these criteria.
[0005] FIG. 1 of the state of the art represents diagrammatically a
device implementing an optical fiber coupled to an integrated
optical circuit with a conventional Y-junction. The guided optical
wave coming from the optical fiber 1 is injected at the interface 2
at the input to the integrated optical circuit whereof the optical
guide is manufactured on a planar substrate 3. The optical wave
coupled to the interface 2 between the media 1 and 4 propagates
first of all through a common optical guide 4 or common trunk 4
before passing through a "taper" zone 5, which will be called
therebelow conical zone, and whereof the width varies gradually
from We toward 2Ws+D. This conical zone 5, ending at a
discontinuity 6 between diverging independent branches 7 and 8,
forms a geometrical preparation at the optical separation between
the optical guides of the branches 7 and 8 called respectively
upper and lower branches, oriented by an angle .+-..alpha. relative
to the propagation axis of the guide in its common portion or
common trunk 4. The discontinuity 6 of width D is a consequence of
the limitations in resolution of the manufacturing technological
processes. In practice, this discontinuity has sizes of the order
of the micrometre possibly tenth of micrometre.
[0006] In a conventional Y-junction device as that represented on
FIG. 1, the mechanism of the losses and optical instabilities
appears mainly in three locations. First of all at the interface 2
because of the shape dis-adaptation between the input optical
field, that of the fibre 1, and the fundamental optical mode
supported by the common trunk 4. Then on the discontinuity 6 where
the fundamental optical mode may be coupled to the higher order or
radiating optical modes because of the discontinuity 6 in
propagation. Finally, along the branches 7 and 8 because of the
separation angle 2.alpha. between both branches.
[0007] The losses at the interface 2 may be increased by using an
adaptation "taper" between both optical modes. As regards the
stability in the common trunk 4 preceding the separation between
the branches, the width We is adjusted so as to support only the
fundamental optical mode. As explains the article of A. Klekamp, P.
Kersten and W. Rehm, "An improved single-mode Y-branch design for
cascaded 1:2 splitters", Journal of Lightwave Technology, vol. 14,
n.degree.12, December 1996, the balance stability at the Y-junction
output is increased while reducing the width of the common trunk 4
on a certain rectilinear portion called monomode spatial optical
filter. Indeed, in the case of an imperfect injection, the external
optical wave coming from the fibre 1 being injected with an angle
and a transversal overlay relative to the common trunk 4, a
fraction of the upper optical mode of order 1 (anti-symmetrical)
may be excited with the fundamental optical mode in the common
trunk 4. If the width We is sufficiently small for the optical
guide to be strictly monomode, the spurious optical mode is
diffracted in the substrate 3 during the propagation in the common
trunk 4 before reaching the conical zone 5. The structure is then
more stable in actual conditions of use.
[0008] At the discontinuity 6 and the branches 7 and 8, the optical
losses and the stability of the Y-junctions may be increased if the
separation angle becomes very small (typically 0.1.degree.) since
the adiabatics of a Y-junction are sensitive to the separation
angle 2.alpha.. When this angle approximates 0.degree., the optical
behaviour of the junction stabilizes and the optical losses
diminish. However, this reasoning does not respect the space
requirement criterion since the lengths of the branches necessary
with such angles to provide sufficient separation of the branch
optical guides are rapidly prohibitive. In practice, a 14.3 mm
longitudinal space requirements should be accounted for a
0.1.degree. angle and a 25 .mu.m final separation between the
centres of the output branches. Moreover, the discontinuity 6 in
the conical zone 5 generates an excitation of the higher order
optical modes detrimental to the stability of the Y-junction, this
even in the case of an adiabatic aperture. Indeed, the end portion
of the conical zone 5 is still characterized by a significant guide
width 2Ws+D capable of supporting several modes of higher optical
order.
[0009] To limit such problem, it has been suggested in the patent
application WO97/32228 a solution which is represented on FIG. 2 of
the state of the art. The Y-junction is characterized therein by an
overlay of the discontinuity 6 toward the inside of the conical
zone 5 by creating an aperture 9 having a lower index than the core
of the optical guide. However, this solution does not suppress
satisfactorily the excitation of the higher order optical modes and
the optical losses. Moreover, the geometrical space requirements is
not reduced relative to a conventional Y-junction.
[0010] For improvement purposes, the solution presented into the
patent application WO02/071112 of D. Sciancalepore and S. Renoldi
and represented on FIG. 3 of the state of the art, offers to
truncate the conical zone in its first portion qui is a source of
optical instabilities and of useless space requirements according
to the authors. With the latter solution, the discontinuity 6
coincides with the transition zone between the common trunk 4 and
the branches 7 and 8. The widths of the branches 7 and 8 at the
input to the conical zone and the parameters D are adapted so as to
create a continuity of the optical fields between the common trunk
4 and the input of the conical zone 5. It is explained in
WO02/071112 that the discontinuity of the optical guide appears at
a portion studied for being monomode, which increases the stability
of the Y-junction.
[0011] In spite of this improvement and all the precautions taken,
the discontinuity between the common trunk 4 and the conical zone 5
remains a potential source of losses and of optical instabilities
by coupling on radiating optical modes and higher order optical
modes.
[0012] A number of other documents of the field of the invention
are also known.
[0013] Thus, document EP-0716336A and the article of VINCHANT J. F
and al. <<INP DIGITAL OPTICAL SWITCH GUIDED-WAVE PHOTONIC
SWITCHING" IEE proceedings J. optoelectronics Vol. 140 N. 5 part J
p. 301-307 show a structure with discontinuous optical guides and
with an input common trunk.
[0014] Document JP-11-352347 shows an optical structure whereof the
optical guides must support both first optical modes, involving a
dependence on the wavelength and the polarization contrary to the
present invention which will be presented below and wherein an
adiabatic modal evolution of the fundamental mode is implemented
and not a coupling between both first optical modes as in this
document.
[0015] Documents JP-04-355714 and DE-4225085 still appear based
upon a coupling of optical modes same as for the previous
document.
[0016] Finally, document JP-2000-180646 shows a structure with
discontinuous waveguides or with other complex conformations
involving optical losses in particular by diffraction.
[0017] The present invention offers to suppress this discontinuity
problem which is responsible on the one hand for optical losses and
on the other hand optical instabilities in the Y-junction. The
solution suggested consists in translating the discontinuity
encountered at the conical zone toward and at the interface making
the connection with the external medium, hence on the input to the
optical guiding common trunk. In such a case, the common trunk
forming the initial trunk of the Y-junction forms a preform at the
optical separation, which guarantees by a geometrical construction
a continuity of the optical field propagated at the separation of
the branches.
[0018] Thus, the invention relates to an optical integrated circuit
with waveguide separation on a substrate, the circuit comprising at
least one optical separating unit, the unit comprising an optical
input/output interface intended for being in relation with an
external means for guiding a light wave, the interface extending in
the circuit through an optical guiding input section of determined
length L1 extended by at least two optical guiding branches
mutually spaced apart substantially symmetrically relative to the
general direction of the input section.
[0019] According to the invention, the input section includes as
many optical guides as there are branches, (each branch extends
from an optical guide of the input section) the optical guides of
the input section being substantially rectilinear and mutually
parallel, two adjacent optical guides of the input section being
separated by an aperture of determined width D, the refractive
index of the opening being lower than that of the optical guides,
each optical guide of the input section having a determined width
We1, and each branch optical guide exhibits a width increasing in
the direction away from the input section from the width We1 up to
a determined width Ws.
[0020] In various embodiments of the invention, the following means
which may be used single or according to all technically possible
combinations, are used: -the unit includes moreover a transition
zone of length L0 between the interface and the input section,
wherein the transition zone includes optical guides extended from
those of the input section, each of the optical guides of the
transition zone having a width increasing in the direction away
from the interface from a determined width We0 up to the width We1,
and in that the opening between two adjacent optical guides of the
transition zone has a width increasing in the direction away from
the interface from a determined width D' up to the width D. [0021]
the widths We1 and Ws are equal, the optical guides of the branches
having constant widths along their paths, [0022] the variation in
width of the optical guides of the branches is linear in relation
to the distance of propagation. [0023] the length L1 of the input
section ranges between 0 and 10 mm. [0024] the semi-angle .alpha.
for separating the branches ranges between 0.1.degree. and
0.50.degree. and preferably about 0.175.degree., (the angle for
separating the branches is 2.alpha.) [0025] the external guiding
means is an optical fiber bonded to the interface of the integrated
optical circuit, [0026] the substrate of the optical integrated
circuit is selected among glass, a semi-conductor, a polymer, a
ferroelectric material in particular lithium niobate (LiNbO3) or
lithium tantalate (LaTiO3), [0027] the substrate of the optical
integrated circuit is selected lithium niobate (LiNbO3) or lithium
tantalate (LaTiO3), [0028] the substrate of the optical integrated
circuit is made of lithium niobate (LiNbO3), [0029] the substrate
of the optical integrated circuit made of lithium niobate (LiNbO3)
is in X cross-section, [0030] the substrate of the optical
integrated circuit made of lithium niobate (LiNbO3) is in Y
cross-section, [0031] the substrate of the optical integrated
circuit made of lithium niobate (LiNbO3) is in Z cross-section,
[0032] the optical guides have been obtained by a titanium
diffusion technique in a substrate of the circuit made of lithium
niobate (LiNbO3), [0033] the optical guides have been obtained by a
proton exchange technique in a substrate of the circuit made of
lithium niobate (LiNbO3), [0034] the optical guides have been
obtained by a titanium diffusion technique in a substrate of the
circuit made of lithium niobate (LiNbO3) with an X cross-section,
[0035] the circuit is intended to operate in forward direction as a
separator, [0036] the circuit is intended to operate in forward
direction as a recombinator, [0037] the circuit is intended to
operate in forward direction and/or in reverse direction as a
separator/recombinator, [0038] the circuit is a Y-shaped optical
separator/recombinator with two branches, [0039] the circuit is a
Y-shaped optical separator/recombinator with three branches, one of
the branches being central on the axis of symmetry of the optical
guides of the circuit, [0040] the circuit is a Y-shaped optical
separator/recombinator with four branches, [0041] the optical
circuit comprises a separating unit and it is a Y-shaped optical
separator/recombinator with at least two branches, [0042] the
optical circuit comprises two cascaded, head to tail mounted
separating units and it is an integrated Mach-Zehnder
interferometer with at least two branches, [0043] the optical
integrated circuit comprises two cascaded, head to tail mounted
separating units and it is an integrated Mach-Zehnder
interferometer with at least two branches, [0044] the optical
integrated circuit comprises two cascaded, head to tail mounted
separating units and it is an integrated Mach-Zehnder
interferometer with four branches, [0045] the optical circuit
comprises two cascaded, head to tail mounted separating units,
electrodes in relation with the optical guides of the branches and
it is an integrated Mach-Zehnder interferometrical modulator with
at least two branches.
[0046] The advantages associated with the use of a junction
according to the invention are multiple. First of all, reduction of
the sources of losses and of optical instabilities relative to a
conventional Y-junction: both sources of losses and of optical
instabilities which are generated firstly at the insertion of an
external optical signal in the input optical guide, hence at the
interface, then secondly at the discontinuity into the conical zone
in conventional circuits, are gathered thanks to the invention into
a single source of optical losses, at the insertion and then
constitutes a single optimization problem of the interface.
Secondly, the optical structure suggested may advantageously
improve the balance of the Y-junction. Indeed, the guiding input
section which is formed of two (at least) substantially parallel
and straight optical guides forms an optical superstructure hence
the modal properties are equivalent to those of a single optical
guide of width smaller than the sum of the widths of both optical
guides taken separately. The straight input section acts hence as a
monomode spatial optical filter which suppresses the spurious
optical modes resulting from an imperfect injection between the
external signal and the input guide and which are liable to modify
the optical balance of the Y-junction. Thirdly, in terms of
longitudinal space requirements, the gain is also noticeable since
the adiabatic conical zone has disappeared to leave room for direct
separation. The expression of the gain in space requirements
relative to a conventional Y-junction is calculated according to
the following formula: X=(D+We1)/(2.tan .alpha.), and it can be
noticed that the gain in room increases when the separation angle
decreases. For instance for a separation semi-angle
.alpha.=0.15.degree., the longitudinal gain relative to a
conventional Y-junction is of the order of 22%. The invention
suggested enables hence to improve simultaneously numerous quality
criteria of a Y-junction.
[0047] The present invention will now be exemplified without being
limited thereto with the following description in relation with the
figures below:
[0048] FIG. 1 of the state of the art representing diagrammatically
a device with an optical fiber at input followed by an integrated
optical circuit comprising a conventional Y-junction,
[0049] FIG. 2 of the state of the art representing diagrammatically
a Y-junction with an aperture in the conical zone,
[0050] FIG. 3 of the state of the art representing diagrammatically
a Y-junction with partial elimination of the conical zone and
optical adaptation between the common optical guides and those of
the branches,
[0051] FIG. 4 which represents diagrammatically a device formed of
an optical fiber at input and of an integrated optical circuit
comprising a Y-junction type separating unit according to the
invention,
[0052] FIG. 5 which represents a device formed of an optical fiber
at input and of an integrated optical circuit comprising a
variation of the Y-junction type separating unit according to the
invention,
[0053] FIG. 6 represents a device formed of an optical fiber at
input and of an integrated optical circuit comprising a
three-output extension of the Y-junction according to the
invention,
[0054] FIG. 7 which represents a first application of Y-junctions
according to the invention in a Mach-Zehnder type
interferometer,
[0055] FIG. 8 which represents a second application of Y-junctions
according to the invention in an optical power separator and
recombinator which may be used in particular in optical fiber
gyrometers,
[0056] FIG. 9 which represents a variation of FIG. 7 with placing
in parallel two Mach-Zehnder type interferometers inside a main
interferometer, and
[0057] FIG. 10 which represents a series of comparative
experimental measurements of optical losses of Y-junctions.
[0058] FIGS. 1 to 3 of the state of the art having been presented
in the introductory section of the present document, the present
invention will now be described in relation with FIG. 4. The
Y-junction type device presented therein is formed by an integrated
optical circuit on a substrate 3. An optical fiber 1 is bonded to
the optical circuit. An optical transition is formed at the
interface 2, which may be perpendicular as represented, preferably
tilted (chamfered injection faces) to eliminate the spurious
reflections between the optical fiber 1 and the ends of the optical
guides 4a and 4b realized in the integrated optical circuit. The
light of the optical fiber 1 which is injected on the input to the
optical circuit at the interface 2 passes through the guides 4a and
4b of the integrated optical circuit and travels a guiding input
section of length L1, both optical guides 4a and 4b in this input
section being substantially straight and mutually parallel and each
of width We1. On the interface 2 appears in the optical circuit a
discontinuity 6 which is at the end of an aperture 9 separating
both optical guides 4a and 4b of the input section, the aperture 9
and hence the discontinuity 6 at the end thereof, have a width D
and optical properties different from those of the optical guides
4a and 4b. The refractive index of the discontinuity 6 and of the
aperture 9 is smaller than that of the optical guides 4a and 4b.
The width We1 of each optical guide 4a and 4b and the spacing D are
adjusted so that the superstructure formed by the assembly of both
guides 4a and 4b can only guide the fundamental optical mode at the
working wavelength. The length of the input section L1 is fastened
to suit the needs of stability and optical balance desired at
output of the Y-junction. It may vary from 0 to several
millimetres. Following the input section of length L1 lies zone for
separating the branches formed by the gradual spacing away of an
upper branch 7 and of a lower branch 8 of optical guides which are
oriented by an angle .+-..alpha. relative to the axis of symmetry
carried by the general direction of the input section. The optical
guides forming the branches 7 and 8 come apart from one another and
widen gradually each to reach a final width Ws either to meet an
output face of the integrated optical circuit, or to carry on their
own paths with this width Ws. More generally speaking, the upper 7
and lower 8 branches represent the extension of the optical guides
4a and 4b and the separation zone may be interpreted as an
evolution of the input optical superstructure where the internal
parameters (widths of the optical guides and spacing between both
optical guides) evolve during the propagation. In the case
presented on FIG. 4, the evolution of the widths of the optical
guides and of spacing between the optical guides is linear but in
non-represented variations, non linear evolutions of the widths
and/or spacing are considered.
[0059] The profile of the separation zone is designed so as to
limit to the maximum, on the one hand the optical losses due to the
coupling of the fundamental mode on the continuum of the radiating
modes and on the other hand to the instabilities due to the
coupling on higher order modes. The behaviour of light in such
optical structures may be interpreted from the evolution of the
optical supermodes. In this type of structures, the guided and
radiating optical modes are linked by coupled propagation
equations. The coupling coefficients of these equations are
approximated by simplification of the continuous separation into a
discrete series of abutting rectilinear sections. In such a case,
the coupling coefficients are proportional to the overlaying
integral between the optical modes at each elementary transition
between the sections:
I.sub.mn.sup.i.fwdarw.i+1=.intg. E.sub.m.sup.iE.sub.n.sup.i+1
where E.sub.m.sup.i characterizes the amplitude of the m order
optical mode into the section i, E.sub.n.sup.i+1 the amplitude of
the n order optical mode into the section i+1 and
I.sub.mn.sup.i.fwdarw.i+1 the overlaying integral linked to the
coupling coefficient of the m optical mode on the n mode.
[0060] It then becomes obvious that the number of optical modes
liable to be guided defines the level of risk of coupling the
energy of the fundamental mode on the higher order guided modes.
The separation is then designed so as never to sustain more than
two optical modes (fundamental and order 1 anti-symmetrical mode).
A digital resolution software of the propagation equations based
upon the `Beam Propagation Method` (BPM) may be used for simulating
and estimating theoretically the optical losses as well as the
possible coupling on higher order modes in this type of
structure.
[0061] In such a case we refer to an adiabatic separation since the
integrality of the energy of the fundamental optical mode at input
is kept throughout the propagation in the separation. At separation
output, the optical power is then shared in equal proportions on
each of the branches 7 and 8. The separation zone, characterizing
the second portion of the Y-junction, is a simple evolution of the
initial optical superstructure where each optical guide widens and
comes away relative to its neighbour. The width and coming-apart
parameters are calculated in relation to predetermined criteria
such as optical losses and space requirements. This second portion
of the Y-junction is then inscribed in the extension of the initial
portion. In other words, the Y-junction of the invention may be
perceived as a symmetrical optical structure whereof the upper and
lower branches are stretched up to the input to the integrated
optical circuit.
[0062] FIG. 5 exhibits a device implementing a realization
variation of an integrated optical separation on an optical
circuit. In this variation, the initial portion of the optical
guides is provided with a zone for gradual adjustment of the width
and of the distance between them so as to find the best compromise
between the optical losses by dis-adaptation of shape with the
external signal coming from an optical fibre for instance, the
propagation losses and the monomode spatial optical filtering. The
latter structure stems from the hypothesis that the geometrical
conditions of the optical superstructure promoting optical
overlaying at the interface between the fibre and the integrated
optical circuit will not always match the optimal geometrical
conditions of propagation optical losses in the initial portion of
the Y-junction. The light coming from an optical fiber 1 is
injected at the input to the integrated optical circuit on a
substrate 3 at the interface 2 forming the transition zone. The
initial optical superstructure preceding the spacing-apart zone of
both branches is divided into two sections, first section of length
L0 then second section of length L1. The first section is a
transition zone which acts as an optical adaptor between the
optical signal derived from the fibre 1 and the guided fundamental
optical mode of the second section. This first section is formed of
two upper 10a and lower 10b optical guides matching those
respective ones of the second section, then, their respective
branches and whereof the widths vary of We0 at the interface 2
forming the transition zone, to We1 at the input to the second
section. In parallel, the aperture 9 between the guides sees its
value evolve from D' at the interface 2 forming the transition zone
to D at the input to the second section (D'<D). The initial
dimensions of the optical superstructure, We0 and D', are adapted
in order to minimize the insertion optical losses with the external
signal coming from the optical fibre.
[0063] Optimization consists in maximizing the covering rate
between the distributions of the electrical fields at the interface
2:
.eta. = .intg. E 1 E 2 * .intg. E 1 2 .intg. E 2 2 ##EQU00001##
where E.sub.1 and E.sub.2 correspond respectively to the amplitude
of the optical mode of the fibre and to the amplitude of the
fundamental optical mode of parameters We0 and D'. The values are
restricted between 0, in the worst case, and 1, in the case of a
perfect superimposition between the optical fields. The optical
modes and the covering rate may be calculated using a digital
resolution software based upon the finite difference method.
[0064] The second section is used for filtering spatially the
spurious optical modes liable to be energized by injecting light
from the optical fiber while confining sufficiently the fundamental
optical mode so as not to generate additional optical losses. This
second section is formed of two parallel optical guides which will
match their respective upper 4a and lower 4b branches. The length
L1 and the parameters We1 and D characterizing the monomode spatial
filter are calculated so as to minimize the instabilities such as
the optical unbalance between the branches 7 and 8.
[0065] The corresponding optimization process makes use of digital
simulation tools such as the `beam propagation method`. The
principle consists in reproducing an imperfect injection of light
by offsetting by a few micrometres the transversal centre of the
fibre relative to the centre of the injection guide. The unbalance
at output between the arm 7 and 8 is then calculated by the formula
P7/(P7+P8) or P8/(P7+P8) then a limit is set according to the room
available on the circuit and the alignment tolerances. It can be
noted however that the greater the distance L1 the better the
balance between the output arms.
[0066] In the separation zone of the branches 7 and 8, the optical
guides associated with the upper 7 and lower 8 branches come apart
while being oriented by an angle .+-..alpha. relative to the axis
of symmetry of the optical guides of the circuit. Their respective
widths evolve from We1 to Ws as previously.
[0067] Thus, the invention in its general concept relates to an
optical integrated circuit on a planar substrate comprising optical
guiding means forming at least one optical separation called
Y-junction. The optical guiding means of the initial portion of the
Y-junction are formed of many parallel optical guides as branches
at output of the Y-junction. This initial portion preparing the
separation of the branches forms an optical superstructure whereof
the geometrical features, such as the width and the distance for
separating the parallel optical guides, are selected so as to
verify the conditions of monomodality and adaptation to the optical
signal from the outside to the integrated optical circuit.
[0068] The principle for separating the optical guide developed for
a Y-junction fitted with an inlet and two outlets may be
extrapolated to an inlet and multiple outlets within the limit of
the resolutions reachable by the technology. In order to illustrate
the extrapolation principle to N outputs of the invention, an
example is given on FIG. 6 with a device called separator 1 toward
3. The external signal derived from the optical fiber enters the
integrated optical circuit 3 through the optical superstructure
composed of three parallel optical guides. The structural
parameters such as the width and the spacing between the optical
guides are adapted so as to render the optical superstructure
compatible with the optical mode of the input fibre and so that the
circuit only supports the fundamental guided optical mode. In all
the possible variations it should be borne in mind that the
performed structure is essentially symmetrical.
[0069] The device exemplified until now relates to an optical
integrated circuit on a substrate with at least one Y-junction
operating in forward direction, i.e. as an optical guide separator,
but the principle remains applicable to a Y-junction operating in
reverse direction, i.e. as an optical guide recombinator. Thus, the
invention is liable to find numerous practical applications in
integrated optical circuits with the most diverse functionalities
and several examples are given on FIGS. 7, 8 and 9.
[0070] FIG. 7 exhibits a first example of application with an
optical integrated circuit on a substrate with two Y-junctions of
the invention arranged in cascade, head to tail, and connected to
one another. The first junction acts as a divider (separator) of
optical power while the second acts as an optical power
recombinator. The resulting global optical structure forms an
interferometer called integrated Mach-Zehnder interferometer. In
such a case, the invention offers an additional advantage with
respect to the conventional Y-junctions since the monomode spatial
optical filter which partook of enhanced stability in forward
operation may prove useful, in reverse operation, of enhanced
extinction rate. The radiating anti-symmetrical optical mode at
recombinator output is diffracted more easily outside the optical
guide thanks to a smaller equivalent guide width.
[0071] FIG. 8 exhibits a second example of application with an
integrated optical circuit which may be used as a
divider/recombinator, in particular pour applications in optical
fibre gyrometers based upon the Sagnac interferometric effect.
[0072] FIG. 9 exhibits a third example of application with an
integrated optical circuit which uses as a main base the circuit of
FIG. 8. The optical structure is nevertheless more complex since
each branch of the first Mach-Zehnder is the seat of an second
structure of the same type obtained by splitting.
[0073] These examples with cascading several Y-junctions and/or
splitting are a simple demonstration of the implementation
possibilities of the invention.
[0074] It should be noted that as the geometrical dimensions of the
Y-junction being adaptable, the integrated optical circuit of the
invention may apply to any optical wavelength as long as light is
guided in the circuit.
[0075] Among the possible modalities for realizing the optical
integrated circuit the use of any type of substrate may be
considered as long as the latter is liable to support optical
guides. Thus, the invention applies, among other things, to optical
circuits integrated on glass, on semi-conductors, on polymers, on
ferroelectric materials such as lithium niobate (LiNbO3) or lithium
tantalate (LaTiO3). The particular case which is presented by way
of example relates to the application of the invention to an
integrated optical circuit on a lithium niobate substrate.
[0076] Lithium niobate is a crystal used currently for the
manufacture of active integrated optical circuits based upon the
electro-optical or acousto-optical effect. Among its properties,
one will observe that it belongs to the class of uniaxial
birefringent crystals: the crystallographic axis Z corresponds to
the optical axis, noted as extraordinary axis with index Ne, while
both other axes, X and Y correspond to the ordinary axe with index
No. In practice, the optical guides are manufactured in surface on
substrates which may have the three main crystallographic
orientations, i.e. the so-called `X sectional` substrates, whereof
the axis X is perpendicular to the surface, so-called `Y
sectional`, whereof the axis Y is perpendicular to the surface,
which are known for their temperature stability, or so-called `Z
sectional`, whereof the axis Z is perpendicular to the surface,
which are characterized by depending strongly on the
pyro-electrocal effect but also by better electro-optical
efficiency.
[0077] Currently, two technologies applied at industrial scale
enable to manufacture optical guides on lithium niobate: titanium
diffusion and proton exchange. It should be noted that other less
conventional manufacturing techniques such ion implantation or
lithium niobate etching may also suit the invention.
[0078] Titanium diffusion is a method consists in raising locally
the refractive indices Ne and No by doping the crystallographic
array of the host, in this case LiNbO3. The doping is realized by
very high temperature thermal diffusion, typically of 900.degree.
C. to 1150.degree. C. Proton exchange is a method consisting in
local elevation of the extraordinary index Ne via a substitution
chemical reaction. By making lithium niobate contact hot acid, pure
or diluted, the lithium ions of the crystal close to the surface
are gradually replaced with protons. After the exchange operation,
the substrate may be annealed for softening and stabilizing the
index profile.
[0079] In order to check the validity of the suggested invention,
several integrated optical circuits comprising different
Y-junctions have been realized on lithium niobate. The pattern of
the optical circuit selected corresponds to that of FIG. 7, i.e. a
Mach-Zehnder interferometer. The optical guides have been obtained
by the titanium diffusion technique on an X-sectional lithium
niobate substrate.
[0080] The different optical circuits have been realized within a
single board so as to guarantee identical treatment. The
geometrical parameters such as We1, D and Ws have been set on the
basis of previous tests and in the present case only the separation
angle and the type of Y-junction vary from one circuit to
another.
[0081] FIG. 10 gives the optical losses per Y-junction upon
completion of the tests at the wavelength of 1550 nm. The estimate
of the optical losses per Y-junction is obtained by subtracting the
optical losses of the Mach-Zehnder interferometers from the optical
losses of reference rectilinear guides then by dividing by the
number of Y-junctions, two in the present case. The curve as a
dotted line with the squares corresponds to the measuring results
on conventional Y-junctions having a semi-angle .alpha. for
separation .alpha.=0.5.degree.. The losses per junction are
estimated in average as 0.99 dB. The continuous curve with the
circles represents the measurements of conventional Y-junctions
having a semi-angle .alpha. of smaller separation, typically
.alpha.=0.2.degree.. The losses per Y-junction are estimated in
average as 0.58 dB. The curve with intermittent dashes with the
triangles is associated with the measurements made on the
Y-junctions of the invention with a semi-angle .alpha. for
separation .alpha.=0.175.degree.. The average losses amount to 0.31
dB per junction.
[0082] Table 1 below gives a comparison of the performances of the
Y-junctions manufacture on the base of the selection criteria such
as geometrical space requirements, optical losses and, indirectly,
monomode spatial optical filtering by the extinction rate:
TABLE-US-00001 TABLE 1 Y-junction conventional Y- conventional Y-
of the junction junction invention .alpha. 0.5.degree. 0.2.degree.
0.175.degree. Longitudinal space -60% 0% -11% requirements Optical
losses 0.99 dB 0.58 dB 0.31 dB Extinction rate 25 dB 22 dB 25
dB
[0083] In this table 1, the conventional Y-junction with
.alpha.=0.2.degree. is selected as reference Y-junction for space
requirements.
[0084] The conventional Y-junction with a semi-angle .alpha. of
0.5.degree. enables to save 60% made of longitudinal space
requirements but to the detriment of optical losses. Conversely,
the Y-junction of the invention enables to save 11% on space
requirements even with a smaller separation semi-angle
(0.175.degree. instead of 0.20.degree.). The optical losses also
decrease relative to the reference.
[0085] The experimental results show that the Y-junction of the
invention enables to save simultaneously on space requirements and
optical losses whereas a conventional Y-junction may only improve
one of both criteria at a time. With equivalent space requirements,
the results obtained show the efficiency of the Y-junctions of the
invention. The performances displayed reflect combined improvement
of the fibre/guide coupling efficiency (gain of 0.15 dB obtained by
optical simulation), a reduction in the radiating optical losses by
suppressing the discontinuity of the optical guide at separation
and by minimization of the separation angle 2.alpha. between both
branches.
[0086] In the Mach-Zehnder type intensity modulators, the
extinction rate translates the efficiency of the monomode spatial
optical filtering of the foot of the Y-junction. This rate is
expressed as the ratio in decibels between the passing and blocking
optical levels. The conventional Y-junctions with a semi-angle of
0.5.degree. are currently used for obtaining high extinction rate
(25 dB). It can be noticed in table 1 that when this angle .alpha.
changes from 0.5.degree. to 0.2.degree., the optical losses do
improve but the extinction rate degrades by 3 dB (22 dB). In order
to regain equivalent performances, the foot of the Y-junction
should be lengthened so that the radiating anti-symmetrical optical
mode is diffracted more efficiently in the substrate. Simultaneous
improvement of the optical losses and of the extinction rate of a
conventional Y-junction implies unacceptable increase in its
geometrical dimensions. The Y-junction of the invention exhibits a
different behaviour since the extinction rate remains high and
equivalent to that of the conventional junction with a 0.5.degree.
separation semi-angle, even with a small separation semi-angle
.alpha.=0.175.degree.. This phenomenon is due to a smaller
equivalent guiding width of the foot of the Y-junction resulting
from the invention. The radiating anti-symmetrical mode is
diffracted more easily in the substrate during its propagation.
[0087] In conclusion, when the aperture angle is fixed, the
Y-junction of the invention proves more performing than the
conventional Y-junction, as well as regards space requirements,
optical losses or monomode spatial optical filtering. The
experimental results obtained on Mach-Zehnder interferometers put
in evidence the impact that the Y-junctions of the invention may
exert and it becomes obvious these deviations in performances will
be increase with the number of cascaded Y-junctions.
* * * * *